The appreciation of an association between the autonomic nervous System and arrhythmic mortality and a series of experimental results represents the rationale for using baroreflex sensitivity - which predominantly measures reflex vagal activity - as a clinical tool for the identification of patients at increased risk. Among different techniques, the measure of the heart rate slowing in response to a blood pressure rise induced by small intravenous boluses of phenylephrine has been the most widely used method for assessing baroreflex sensitivity both in patients with myocardial infarction and with chronic heart failure. Following small size studies, the ATRAMI (Autonomic Tone and Reflexes After Myocardial Infarction) trial, by studying 1284 myocardial infarction patients has definitely demonstrated not only that a depressed baroreflex sensitivity (< 3 msec/mmHg) is a strong risk factor for cardiac death, but also that the information gained by the analysis of autonomic markers adds to the information obtained by better recognized measures of cardiovascular outcome such as left ventricular function and ventricular arrhythmias. In these patients, the analysis of autonomic activity might be of value in the identification of patients who may need an implantable automatic defibrillator for primary prevention of sudden cardiac death. The analysis of baroreceptor reflexes is of prognostic value also in patients with chronic heart failure, particularly those with severe mitral regurgitation.